Many electronic devices and electrical systems, such as transistors, integrated circuits, power controls, switches, microprocessors, and the like, generate heat during operation. The capability of some electronic devices is limited by their ability to remove or expel internally generated heat. This heat must be removed from the device to avoid general or localized thermal degradation or failure of the device. In some devices, the heat generated is sufficiently dissipated to the surrounding environment by the enclosure, package, header, or leads. Other devices require additional apparatus, such as heat sinks, heat exchangers, etc., for removing and dissipating excess thermal energy.
For purposes of the present invention, a heat sink is any body of metal or like material which is placed in thermal communication with an electronic device package or other heat generating component for transferring internally generated heat from the device and for rapidly dissipating this heat to the surrounding environment by conduction, convection, and/or radiation. In order to accomplish this, heat sinks are generally made of materials having high coefficients of thermal conduction such as aluminum, copper, and alloys thereof. Heat sinks may be extruded, machined, molded, sawed, or formed of sheet metal bodies. A typical heat sink for electrical applications functions by conducting heat away from the heat generating component and dissipating the heat into the surrounding air. Accordingly, heat sinks are typically shaped to maximize surface area by incorporating fins or pins. Increasing the heat sink's surface area increases the heat sink's ability to dissipate heat to the surrounding atmosphere.
In order for the heat sink to operate efficiently, it must be secured to, or otherwise placed in good thermal communication with, the heat generating device. Various means have been used to attach heat sinks in thermal communication with heat generating device packages. A known practice is to glue, solder, or otherwise adhere a heat sink directly to a predetermined surface of the body of a heat generating device package with heat-conductive epoxy, solder paste, thermally enhanced adhesives, or the like. Heat sinks may also be mechanically attached to electronic device packages with resilient metal clips mounted on the heat sink or with screws, bolts, clamps, or other connective means which urge the heat sink and electronic device package into mutual physical contact. In addition, heat sinks may be remotely located but thermally coupled to a heat generating device via a heat spreader device, a heat pipe, or any other means of transferring heat from the source of the heat to the heat sink.
Recently, technological advances have allowed electronic components to decrease in size while significantly increasing in power and speed. This miniaturization of electronic components with increased capability has resulted in the generation of more heat in less space with the electronic device packages having less physical structure for dissipating heat and less surface area for attaching a heat sink to dissipate the heat. The reduction of surface area available to attach a heat sink or other heat dissipating device effectively reduces the thermal path for the heat to move from the heat generating device to the heat dissipating device. As used herein, "thermal path" refers to the path along which, or the heat conductive material through which, the heat is transferred from the heat generating device to the heat dissipating device (heat sink). A smaller thermal path means less heat can move from the heat generating device to the heat sink; thus, the heat is dissipated at a slower rate and ultimately less heat can be dissipated. By analogy, a smaller thermal path is similar to a smaller water pipe wherein less water can pass through it; thus, the water is transferred at a slower rate and ultimately less water can be transferred. In order to maximize the efficiency and capability of a heat dissipating assembly, one must seek to design the largest thermal path between the heat generating device and the heat sink so that all of the heat being generated can be dissipated.
Further complicating these general thermal management issues, however, is the growing preference for surface mounting electronic components on printed circuit boards (PCBs) or other substrates. The use of surface mount PCBs is desirable because it is a less costly and less time consuming process of fabricating and populating PCBs than the older manufacturing assembly process which required insertion of components through holes in the circuit board for subsequent soldering operations. Surface mount PCBs allow for the increased use of automated manufacturing and assembly techniques. In particular, surface mountable devices are typically robotically picked and placed on the PCB and then soldered to the PCB in one automated manufacturing process. In addition to reducing assembly costs, however, the surface mount technology has allowed for even greater miniaturization of the electronic device packages used on the boards. These smaller surface mount device packages further reduce the device's ability to dissipate its own heat, thus increasing the need for separate heat sinks. In addition, the smaller packages make it increasingly difficult to attach a heat sink directly to the device package. Finally, even when a heat sink can be attached directly to the heat generating device package, the size of the thermal path is limited by the available contacting surface area on the smaller device package.
Several methods have been suggested to effectively dissipate heat from these smaller surface mount electronic device packages. One common approach is to use the ground plane, or other similar thermally conductive area of the PCB (such as a thermal plane, thermal pad, or thermal land) as a rudimentary heat sink to spread and dissipate the heat directly from the PCB. If the ground plane is used as a thermal plane, heat from the electronic device package can be transferred to the thermal/ground plane via the ground leads of the electronic device package. Additionally, if the electronic device package has a collector tab, or other heat dissipating tab, this tab can be thermally coupled to the thermal plane of the PCB via a thermal pad on the surface of the PCB. Thus, the ground leads or tab of the electronic device package can be used as "thermal leads" to transfer heat from the device package to the thermal plane of the PCB. It should be noted, however, that the heat transferred to the thermal plane of the PCB must eventually be dissipated to the surrounding environment. If the thermal plane and thermal pads are incapable of adequately dissipating the heat to the surrounding atmosphere, a heat sink or other heat dissipating device may still be required. If required, a heat sink can be soldered to a thermal pad in direct, or indirect, thermal communication with the thermal plane. Although the thermal pads and heat sinks may ultimately provide adequate dissipation of the heat generated, these alternatives often consume valuable board space thereby increasing the size of the PCB or limiting the available board space for populating the PCB, both of which are undesirable side effects.